In conventional vacuum forming apparatus and processes, a heated plastic film or sheet is caused to be drawn into the form of a mold by the production or by the introduction of a vacuum beneath the sheet. The films may be polyethylene and polyropylene or similar materials. Air is evacuated from the space beneath the sheet causing the heated sheet to closely conform to the shape of the mold. The sheet so formed is held in this position for an appropriate period of time so it will retain its formed shape upon cooling.
In conventional present-day methods, the removal of air from beneath the heated plastic, thus producing a vacuum, has been accomplished in a number of ways. In one system, vacuum is produced in a relatively large capacity tank--large in comparison to the volume of air that must be removed from beneath the heated plastic sheet in order to cause the plastic sheet to conform to the shape of the mold. The vacuum or negative pressure is then connected to the area beneath the heated plastic sheet and the vacuum forming operation is quickly accomplished.
The disadvantage in this type of system is the requirement of an inordinately large tank resulting in the large cost and the necessity of accommodating the large tank as part of the apparatus.
Another method of accomplishing the desired results is to provide a very large capacity vacuum pump that can be utilized to produce the desired vacuum--i.e. the rapid removal of air necessary to produce the vacuum. The disadvantage of this method is the cost of a vacuum pump necessary to produce the desired vacuum is inordinately high and not an efficient use of the pump's capacity.
A compromise between the two referred to solutions is simply to use a smaller vacuum pump or a smaller tank in which the vacuum is initially produced and before it is connected to the space to be evacuated. However, this takes more time for the vacuum forming operation to be accomplished, resulting in cooling of the heated film and more difficulty forming it into the desired shape.